Highland Ham wrote in
:

I DID however, at around the same time, own a 2 meter Jap all-mode
transceiver that I happened to measure the "S" meter accuracy with an
HP signal generator. It turned out that 2 uVolts was "S"-1. THREE
uVolts was "S"-9.
==============================
Is it correct that for frequencies up to 30 MHz a S9 signal is 50
microvolt into 50 Ohms (or -73 dBm) but that for higher frequencies a
S9 signal is 5 microvolts into 50 ohms (or -93 dBm).
If that is (the agreed) norm ,was it ever formally sanctioned by IARU
?

I can hardly believe that any of the far eastern rice boxes have a
properly calibrated S-meter. Also the top end of the S-meter scale is
usually rather 'compressed', which surprises me since ICs with a log
type input/output relationship must be readily available.

Time to attempt calibrating the S-meter of my (almost vintage) TenTec
Paragon TRX.


Frank GM0CSZ / KN6WH


Frank, leaving aside your apparent prejudice about the country of origin
of a radio...

It seems that IARU Region 1 Technical Recommendation 1 lays down the
Region 1 view of S meter equivalence in power, but that does not seem to
have wider acceptance more than 25 years later.

Talking about SSB telephony receivers...

The next problem that occurs is that many radios are loosely calibrated
with the selectable internal preamp OFF, a good idea for lower HF bands,
but questionable on VHF and above. The user manuals don't often state the
correct configuration for calibrated S meter response. Apparently, the
technology hasn't advanced enough to have the S meter calibrated whether
or not the internal preamp on in use.

(It is quite possible that the example that Harold gave is one of those
radios that is calibrated with internal preamp OFF and he measured it
with preamp ON.)

The radio's S meter may often be fairly roughly calibrated between about
S6 and S9+20 (with the preamp OFF if that is the case for the particular
radio), but is unlikely to be very accurate below and above that range.

Considering the design of a transceiver, if the S meter is calibrated
with preamp OFF, the receiver could have a noise figure around 15dB,
which means its noise floor is around -126dBm. Typically, the AGC is
deferred until the signal reaches about 20dB above the noise floor, so
that would be -106dBm+ which is about S3 and a half+. So the AGC will not
operate, and the S meter not deflect until the signal is above S3+, yet
the meters will typically be scaled from S0 or S1.

Next, these radios may be used with an external preamp, or a transverter,
both with substantial gain. So, a transceiver that is roughly calibrated
with the preamp OFF, is used with preamp ON for another 20dB or so of
gain, then an external 25dB preamp offset by a few dB of line loss, so
the S meter is now reading some 35dB high. The increased gain is usually
much higher for a LAN-transverter-transceiver cascade.

In my experience, quantitative signal reports handed out on VHF and above
are commonly nonsense, which is interesting given the greater focus on
weak signal working and understanding path characteristics.

The other thing that contributes to this nonsense is that people cheating
on power often give the other station the same report that they received
to disguise the lack of symmetry due to their own excessive tx power.

FM receiver S meter calibration is a whole 'nother thing. Lacking an AGC
system, it is typically limiter current that is used to drive the S meter
and it usually takes less than 10uV to drive them to full scale, S9 might
be indicated by just a few uV. Mind you, a 5uV FM signal has very good
S/N, but that doesn't make it a strong signal.

Owen